1. Field of the Invention
The present invention relates to an evoked response detector for a heart stimulator for determining evoked response in the presence of polarization, the heart stimulator having a pulse generator and control means for controlling the pulse generator to produce stimulation pulses of varying amplitudes, and a lead being intended to be introduced into the heart of a patient and connected to the pulse generator for delivering stimulation pulses to the heart, the evoked response detector includes measuring means for measuring the electrode signal picked up by the lead.
2. Description of the Prior Art
To reduce the energy consumption of heart stimulators an automatic threshold search function, a so called AUTOCAPTURE(trademark) function, is used to maintain the energy of the stimulation pulses at a level just above that which is needed to effectuate capture, cf. e.g. U.S. Pat. No. 5,458,623. A reliable detection of the evoked response, which then is necessary, is, however, not a simple matter, especially when it is desired to sense the evoked response with the same electrode as the one delivering the stimulation pulses. This because of the fact that the evoked response potential is small in amplitude compared to the residual polarization charge. The residual charge decays exponentially but tends to dominate the evoked potential for several hundreds of milliseconds after the stimulation. If the polarization is too high, it could be wrongly interpreted by the evoked response detector as a capture, i.e. contraction of the heart. The AUTOCAPTURE(trademark) algorithm could then by mistake adjust the output amplitude of the stimulation pulse to a value below the actual capture level, which will result in no capture. If the used pacing lead has significant polarization this could consequently disturb the autocapture function and result in loss of capture.
U.S. Pat. No. 5,417,718 discloses an autocapture system within an implantable pulse generator that automatically maintains the energy of a stimulation pulse at a level just above stimulation threshold level. The electrical post-stimulus signal of the heart following the delivery of a stimulation pulse is compared to a polarization template, determined during a capture verificastion test. A prescribed difference between a polarization template and the post-stimulus signal indcates capture.
A unipolar sensed evoked response signal differs from a bipolar sensed evoked response signal both in duration and amplitude, see M. W. Baig et al., xe2x80x9cComparison of Unipolar and Bipolar Ventricular Paced Evoked Responsesxe2x80x9d, Br. Heart J. 1992, 68: 398-402. The duration of the evoked QRS complex is a measure of total ventricular polarization time in the area of the heart subtended by a sensing dipole, and it depends on the extension of the dipole. This means that the unipolar evoked response signal has a longer duration than the bipolar evoked response. Today""s evoked response detectors are designed to detect the positive slope of the evoked response signal which occurs within a detection window, typically 15 to about 60 ms after the stimulation pulse, and is not suited for detection of the unipolar evoked response signal.
Thus there is mostly at least one significant slope in the bipolar measured IEGM signal, which makes it possible to discriminate the evoked response signal from slowly varying signals such as polarization signals. Thus in U.S. Pat. No. 5,431,693 a method of verifying capture of the heart by a cardiac pacemaker is described. Observing that the non-capture potential is exponential in form and the evoked capture potential, while generally exponential in form, has one or more small amplitude perturbations superimposed on the exponential wave form, these perturbations are enhanced for ease of detection by processing the wave form signal by differentiation to form the second derivative of the evoked response signal for analysis for the evoked response detection.
Unipolar detection of evoked response signal is, however, not possible by this technique. Abrupt slope changes or superimposed small-amplitude perturbations are levelled out if the measurements are made over the longer distance from AUTOCAPTURE(trademark) function available today requires a bipolar ventricular lead for the evoked response detection, whereas stimulation can be accomplished also in unipolar mode.
It has become apparent that the true evoked response signal amplitude is fairly constant, independent of the stimulation pulse amplitude, i.e. the evoked response signal amplitude does not vary with the amplitude of the stimulation pulse (provided that the stimulation amplitude is above the stimulation threshold), whereas the electrode polarization is approximately linearly dependent on the stimulation pulse amplitude for a constant pulse duration, see European Application 0906768 and the description below in connection with FIG. 1. These circumstances make unipolar evoked response detection possible without measurement of the slope of any part of the signal picked up by the lead.
An object of the present invention is to provide a technique for setting the threshold value for evoked response detection for an evoked response detector, the function of which is based on the aforementioned principles.
This object is achieved in accordance with the principles of the present invention in a heart stimulator having a pulse generator and a control unit which controls the pulse generator to emit at least two stimulation pulses having different amplitudes, with one of these stimulation pulses having a high amplitude that exceeds the stimulation threshold value. The pulses are delivered to a heart via a lead connected to the pulse generator. An evoked response detector is also connected to the lead and includes a measuring unit which measures respective electrode signals picked up via the lead immediately following the aforementioned two stimulation pulses. In order to set an evoked response sensitivity, the evoked response detector includes a calculating unit which calculates the polarization Polhigh for the high stimulation pulse and the measured electrode signal Umeaslow for the lowest possible stimulation amplitude. The evoked response detector also includes an analyzing unit which sets the threshold value ERLimit for the measured electrode signal for evoked response detection according to the equation       ER    Limit    =                              U          measlow                -                  Pol          high                    n        +          Pol      high      
wherein n is a number greater than 1, preferably 1 less than n less than 10, provided that Umeaslow less than Polhigh and |Umeaslowxe2x88x92Polhigh| greater than Y, wherein Y is a predetermined value.
The detector according to the invention can be used also for unipolar detection of evoked response, which is an important advantage since unipolar leads are less complicated to manufacture and have longer working life than bipolar electrodes. Of course, the detector according to the invention can also be used for unipolar detection of evoked response by using bipolar electrodes in an unipolar mode of operation. The measurement could then be performed between the ring electrode and the casing of the heart stimulator, since severe difficulties in relation to the measurements are avoided if the tip electrode, which is used for stimulation, is not used for the subsequent measurements.
In an embodiment of the detector according to the invention the analyzing unit activates an automatic threshold search function of the control unit of the heart stimulator provided the conditions Umeaslow less than POLhigh and the |Umeaslowxe2x88x92POLhigh| greater than Y are fulfilled. Thus in this way the use of heart stimulators, like pacemakers with AUTOCAPTURE(trademark) function, can be extended also to patients with unipolar leads. For sensing evoked response in the unipolar configuration not only low polarization leads can consequently be used with AUTO CAPTURE(trademark) pacemakers but also high polarization leads.
In another embodiment of the detector according to the invention the pulse generator is controlled to deliver a predetermined number of stimulation pulses of each amplitude U1stim, U2stim. The calculating unit calculates the mean or maximum amplitudes of the measured electrode signals U1, U2 picked by the lead after each pulse of the number of stimulation pulses, the mean or maximum amplitudes being used for setting the evoked response threshold value and for determining activation of said automatic threshold search function of the heart stimulator. In this way small variations in the picked up signals are suppressed.